When detection of a flaw such as a crack and a dent is made on an electrical conductor such as a metal plate or a metal tube, an excitation coil and a receiving coil are disposed in the vicinity of a surface of the conductor and an alternating voltage is applied to the excitation coil. In the conductor, an eddy current occurs. In the receiving coil, an electromotive force is induced. Based on the electromotive force in the receiving coil, which is a detection signal, presence or non-presence of a flaw is determined. The detection signal represents the state of a composite or resultant magnetic field that is the sum of a primary magnetic field and a secondary magnetic field, at a position where the receiving coil is disposed. The primary magnetic field is generated by the excitation coil. The secondary magnetic field is generated by the eddy current. Depending on presence or non-presence of a flaw in a path of the eddy current in the conductor, the eddy current and the secondary magnetic field vary, and thus the detection signal of the receiving coil changes.
In a flaw detection method where an excitation coil and a receiving coil are provided independently of each other, there are two manners of positioning the excitation coil relative to a flaw detected surface of an electrical conductor, namely, a manner to have an excitation coil surface parallel to the flaw detected surface of the conductor, and a manner to have the excitation coil surface perpendicular to the flaw detected surface. The former, parallel disposition manner is disclosed in Patent Document 1, for example. The latter, perpendicular disposition manner is exemplified in Patent Document 2.
When a flaw in a metal tube is to be detected, there is known a method where an excitation coil and a receiving coil that are annular are inserted in the metal tube and disposed concentrically. This method is exemplified in Patent Document 3.    Patent Document 1: JP 2003-149209 A    Patent Document 2: JP 2003-344362 A    Patent Document 3: JP 5-52816 A